Piezoelectric ceramics



United States Patent 3,502,598 PIEZOELECTRIC CERAMICS Tsuneharu Nitta, Osaka-fa, Kaneomi Nagase, Kyoto-fu, and Shigeru Hayakawa, Osaka-fu, Japan, assignors to Matsushita Electric Industrial Company Limited, Osaka, Japan No Drawing. Filed July 17, 1967, Ser. No. 653,604 Claims priority, application Japan, Aug. 11, 1966, 41/531,118; Apr. 3, 1967, 42/21,690, 42/21,689; Apr. 18, 1967, 42/25,643

Int. Cl. Htllv 7/ 02; C04b 35/00; H03h 9/00 US. 'Cl. 252-623 2 Claims ABSTRACT OF THE DISCLOSURE Piezoelectric ceramics having a low dielectric constant, high Curie point, and a small temperature coefficient of resonant frequency over a wide temperature range consist, according to this invention, of a single phase solid solution composed essentially of M01 percent Sodium oxide 33.3-54.95

Lithium oxide ODS-21.70

and

Niobium oxide 44.449.95

This invention relates to novel ceramic materials for piezoelectric transducers and more particularly to novel piezoelectric ceramic compositions comprising sodium oxide, lithium oxide and niobium oxide.

The electronic industry has recently required piezoelectric materials suitable for making electromechanical transducers operating at high frequency, for example in a megacycle range of 20-200 mc. It is important for making such transducers that the piezoelectric materials have a high piezoelectric activity, a low dielectric constant, and a small temperature dependency of electromechanical coupling coeflicient.

The conventionally known piezoelectric materials, such as quartz and Rochelle salt, are monocrystalline. However, the occurrence of natural good-quality quartz is decreasing year by year, and synthetic quartz is considerably expensive. On the other hand, while Rochelle salt is quite inexpensively available, it is not heat resistant and therefore has a narrow working temperature range. In addition, these conventional materials are disadvantageous for use in electromechanical transducers because of the necessity of cutting along a certain crystal axis.

' In contrast to the monocrystalline piezoelectric materials, piezoelectric polycrystalline materials are advantageous, being readily configured into desired shape and being suitable for mass production with relative ease.

However, conventional piezoelectric ceramics such as lead titanate-zirconate are usually bound up with a high dielectric constant and are not suitable for use in high frequency transducers in the afore-indicated megacycle range. As the capacitance of the transducer increases, the inductance for resonance should be decreased. However, in the high frequency resonating circuit, the inductance must be extremely low unless the capacitance of the transducer is kept low. In order to realize a low capacitance, it is necessary to reduce the cros-sectional area of the cermaic element or to increase the thickness thereof. However, changing only the size of the ceramic element is not desirable in view of the resultant decrease in band width, energy loss from the transmission line due to diffraction effects, etc. Therefore, the ideal solution for problems due to high capacitance is to use a cermaic of low dielectric constant.

Accordingly, a principal object of the present invention is to provide novel piezoelectric materials in the form of ceramics having dielectric constants much lower than those of the conventional piezoelectric ceramics.

A further object of the present invention is to provide piezoelectric ceramics characterized by low dielectric constant, high Curie point, high radial coupling coefiicient, and small temperature coefficient of resonant frequency over a wide temperature interval. (Measurement of piezoelectric properties was made by the IRE standard circuit, and radial coupling coefiicient was determined by the resonant to antiresonant frequency method.)

These and other objects of this invention will become apparent upon consideration of the following detailed description.

In accordance with the present invention, it has been discovered that ceramic material of a single phase solid solution composed essentially of 33.3-54.95 mol percent of sodium oxide, ODS-21.70 mol percent of lithium oxide and 44.4- mol percent of niobium oxide has a low dielectric constant, a high radial coupling coefficient and a small temperature dependence of resonant frequency over a wide temperature interval.

The component oxides are intimately mixed in the desired composition proportions and fired in accordance with a schedule set forth hereinafter for production of a fired ceramic body.

The raw materials for the ceramic are reagent-grade sodium carbonate (Na Co a high purity lithium carbonate (Li CO and a commercially pure grade niobium pentoxide (Nb O Any compound which is converted to the corresponding sodium oxide, lithium oxide or niobium oxide upon firing can be used as raw material. Batches are ball milled with a small amount of methyl alcohol for intimate mixing and then dried. Usually, they are pressed loosely into pellet form and calcined at 1000 C. for 6 hours in a covered alumina crucible. The calcined product is then powdered thoroughly and a few drops of distilled water are added to the powder. The powder is pressed at 75 0 kg./cm. into the form of discs. These discs are fired at a temperature ranging from 1180 C. to 1320" C. in air for 2 hours. A temperature rising rate of 5 C. per minute is maintained. After soaking at 1180 C. to 1320 C. for 2 hours, they are cooled at furnace power off.

Silver paste is fired on the disc surfaces to form electrodes in per se conventional manner. The disc are polarized by applying D.C. (direct current) voltage while immersed in silicone oil at 100 C. A polarizing field of the TABLE I Composition (mol percent) Electrical Property Dielectric Radial cou- N azO L120 Nb205 constant pling coeflicient 50 0 50 262 0. 092 49. 0. O5 50 203 0. 191 49 1 50 190 0. 19 48 2 50 184 0.20 47 3 50 182 O. 23 46' 4 50 185 0.26 45 5 50 215 0. 29 44 6 50 285 0. 31 43 7 50 321 0.32 40 10 50 253 0.25 36 14 50 118 0.21 34 16 50 110 0.20 33. 3 l6. 7 50 0. 20 32 18 50 102 0. 14

piezoelectric elements employing the ceramic materials in accordance with the present invention are shown as a function of the compositions. The maximum value of the dielectric constant and of the radial coupling coefficient are observed in the ceramics of a single phase solid solution composed of 43 mol percent of sodium oxide; 7 mol percent of lithium oxide and 50 mol percent of nobium TABLE 11 mol percent 33 .3049.95 0.0516."70

These compositions shown in Table II are suitable for use at high frequency in view of the small dielectric constant and the high radial coupling coefficient. For instance, the composition comprising 46 mol percent of sodium oxide, mol percent of lithium oxide and 50 mol percent of niobium oxide shows a dielectric constant of 185. In the compositions comprising less than 33.3 mol percent of sodium oxide, more than 16.7 mol percent of lithium oxide and 50 mol percent of niobium oxide, there appears another phase with hexagonal symmetry in addition to the orthorhombic phase. The fired body existing in such two phases shows a poor piezoelectricity. The compositions comprising more than 49.95

l5 Preferable mol percent of sodium oxide iess than 0.05 mol percent i TABLE III 5 V H Preferable mol percent Na o 333-5495 Li O ODS-21.70 Nb O I 45.0-49.95 50 The compositions shown in Table III gives a single phase solid solution in a rhombohedrai symmetry and show ferroelectricity at room temperature. The ceramic specimens e of these compositions show thedielectric constant within a range of 100 to 80 while the radial coupling coef ficient still remains at a relatiyely high value, as 0.20 to 0.34. The room temperature dielectric constant and radial coupling coeflicient of piezoelectric elements employing the ceramic materials in accordance with the present in- 4 vention are shown as a function of compositions in Table IV.

TABLE IV Composition Electrical property (mol percent) Dielectric Radial cou- NazO LizO A Nb205 Constant pling coefficient 44. 00 6. 001 50. 00 285 0. 31 43. 78 6. 4T 49. 75 98 0. 32 i 43. 12 7. 88 49. 0O 86 r 0. 34 41. 36 11.64 47. 00 94 0. 34 40. 48 13. 52 46. 00 100 '0. 28 38. 75 17. 25 44. 00 84 0. 13

Table IV shows that thegcompositions within a range of Table III can exhibit an excellent piezoelectricity.

Moreover, according to this invention, a remarkably lower dielectric censtant can be obtained by employing :the compositions shown in Table V. L

, TfBLE V i Preferable mol percent Na O -Ji 333-5495 005 21.70 4.4445.0

The compositions shown in Table V give a single phase solid solution with rhomboheciral symmetry and show ferroelectricity. Table VI shows the room temperature dielectric constant and radial cpupling coefficient of the piezoelectric ceramic inaterials according to this embodimerit of the present invention.

TABLE VI Composition 'Electrieal property Radial Dielectric coupling N320 LizO NbzO5 constant eoeflicicnt TABLE VII (percent) tr(260 C.)ir( C.)

, Composition (mol percent) Crystal Curie No. at point; X100 N340 LlzO Nb205 25 0. 0.); fr(40 C.)

48 2 t 50 O 345 1.30 7 46 *1 50 O 390 -0.' 85 1 .5 0 410 1. 12 44 B 50 O 420 -0. 76 i 42 8 50 O 436 0. 43 39 11 50 O 442 0. 45 45. 8 6. 3 47. 9 R 470 0. 18 46. 7 6. 4 46. 9 R 360 0. 19 Z 41. 36 11. 64 46. 0 R 480 0. 21 42. 2 9. 8 48. 0 13- 372 *0. 32

It: Rhomb ohedral.

As seen in Table VII, the Curie points of the compositions in accordance with the present invention are in a relatively high temperature range of 320 C. to 480 C. and the temperature coefficients of the resonant frequency is less than 1.8% over the wide temperature range of 40 C. to 260 C.

The piezoelectric compositions according to the present invention can be used for making electromechanical transducers for high frequency use, for example, solid ultrasonic delay lines and filters.

Having thus disclosed the invention, what is claimed 1. A piezoelectric ceramic composition consisting essentially of a solid solution of I Mol percent Sodium oxide 33.3-54.95

Lithium oxide ODS-21.70

and

Niobium oxide 44.4-49.9'5

OTHER REFERENCES Cape K: Dissertation Abstracts, vol. 22, p. 3562 Krainik: Chemical Abstracts, vol. 53, p. 5792c (1959).

HELEN M. MCCARTHY, Primary Examiner 15 J. COOPER, Assistant Examiner US. Cl. X.R. 106-39 

